Step stool



FIG. 1 is a top perspective view of a step stool in accordance with the present design;

FIG. 2 is a left side view thereof;

FIG. 3 is a right side view thereof;

FIG. 4 is a front view thereof;

FIG. 5 is a back view thereof;

FIG. 6 is a top view thereof; and,

FIG. 7 is a bottom view thereof.

The broken lines depict portions of the step stool that form no part of the claimed design. 

CLAIM The ornamental design for a step stool, as shown and described. 